Water pollution is mainly caused by human activities such as agriculture, fish rearing and industrial production. Biological decomposition by using aerobic or anaerobic organisms is claimed to be one of the most ‘green’ methods to treat wastewater. However, inorganic compounds such as nitrates and phosphate ions still remain in the treated water after these conventional methods, which will cause environmental problems such as eutrophication or ground water contamination. U.S. Pat. Nos. 4,333,263 and 5,097,795, disclose an algal turf scrubber using macroalgae for removing the excessive nutrients from waste water. These prior arts relate to immobilized systems which have been shown to accumulate more heavy metals than a free cell system (Brouers et al., 1989; Darnall et al., 1986; Khummongkol et al., 1982; Aksu, 1998). Chevalier (1985) has shown that the uptake of nitrogen and phosphorus by immobilized Scenedesmus sp. is greater than uptake by the free cells of the same organism. In a comparison among different strains of algae, a recent study by Johnson et al. (2010) showed that microalgae in general are more effective than macroalgae in removing the inorganic compounds from wastewater.
Another limitation for algae wastewater purification is light intensity. For example, U.S. Pat. Nos. 6,355,172 and 7,682,503 disclose the use of algae to remove metals from sewage; however both inventions are required to be immersed in water. The high turbidity in the fluid phase is the determining factor of blocking the penetration of natural light into sewage such that the fluid at the middle or bottom level may not have sufficient light intensity for algae to carry out photosynthesis, even though the fluid phase is separated from the solid phase and the sludge in a pre-settling process.
Converting unsuitable wavelengths into suitable wavelengths for photosynthesis is one way to enhance the light intensity by using luminescent materials. In U.S. Pat. No. 6,883,271, a device that converts UV light into growth-enhancing light for growth of plants or vegetables is disclosed. However, such a device is limited to the conversion of UV light and is unable to convert a wide range of non-visible light into a specific wavelength of visible light for specific photosynthetic organisms. A more flexible and compatible light-converting device is required.
In addition to wastewater treatment, algae are also good candidates for alternative energy because their by-product and/or biomass can be converted into biofuel. For example, oils derived from triacylglycerols in oil seed plants (e.g. soybean, sunflower and oil palm etc.) (Durrett et al., 2008) or microalgae (Hu et al., 2008) can be made into biodiesel. Algae is more preferable as a source of biofuel since a recent study reveals that algae have inherent advantages over other sources of biofuel such as higher yield, more rapid cell division and better quality (Robert, 2009).
Therefore, there is a need in the art for an improved biological purification system to remove unwanted inorganic compounds from wastewater by effectively using natural light.